The phenomenon of spiralling or helical multi-lobe formation in holes, produced by the BTA (Boring and Trepanning Association) machining, is experimentally investigated for the solid boring tool. The causes leading to spiralling are deduced from this investigation. The experimental approach pursued in exploring the problem involved the running of the machine, at analytically predicted critical speeds and observing the reoccurrence of the phenomenon. It has been established that sprialling is caused by defectiveness of the tool (radial oversize of the circle-land with respect to the leading pad around the circumference) and the coincidence of the lateral natural frequency of the boring bar-tool asssembly, with five cycles per revoution of the tool, relative to the workpiece. It has also been established that spiralling occurs only in five lobes for the commercially available BTA-solid tool and is a consequence of the standard position of the circle-land, relative to the leading pad. The trials are repeated a number of times with various workpiece materials, to assert validity of the observations.

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